Dielectric ceramic composition, preparation method therefor, and dielectric resonator

ABSTRACT

There is provided a dielectric ceramic composition containing metal elements Ca, Ti, La, Zn and M which is at least one metal element selected from the group consisting of W, Ni, Co, Zr, Sn, Mn and Cu, and represented on a molar fraction basis by the following composition formula: 
     
         aCaO·bTiO.sub.2 ·cLa.sub.2 O.sub.3 
    
      ·dZnO·eMO x   
     wherein 1≦x≦3, and a to e are molar fractions which are each set within a predetermined range. The composition has a high dielectric constant, a high Q value and a stably low resonant frequency temperature coefficient τf.

FIELD OF THE INVENTION

The present invention relates to a dielectric ceramic composition having a high Q value in a high frequency region such as of microwave and millimeter-wave and, more particularly, to a dielectric ceramic composition which can be used for various resonator materials, MIC dielectric substrate materials, dielectric waveguide materials, multilayer ceramic capacitors and the like for use in a high frequency region such as of microwave and millimeter-wave. The invention further relates to a preparation method for such a dielectric ceramic composition and to a dielectric resonator.

BACKGROUND OF THE INVENTION

Dielectric ceramic materials have been used extensively for dielectric resonators, MIC dielectric substrates, waveguides and the like in a high frequency region such as of microwave and millimeter-wave. The dielectric ceramic materials are generally required to have the following three principal properties:

(1) A high dielectric constant to meet a demand for size reduction of devices, because the wavelength is reduced to 1/εr^(1/2) in a dielectric;

(2) A small dielectric loss in the high frequency region, i.e., a high Q value; and

(3) A small change in resonant frequency with respect to a temperature change, i.e., a stably low temperature dependence of the dielectric constant.

Exemplary dielectric ceramic materials hitherto known include oxide ceramic materials such as BaO--TiO₂ based materials, BaO--REO--TiO₂ (wherein REO is an oxide of a rare earth element) based materials and MgTiO₃ --CaTiO₃ based materials (see Japanese Unexamined Patent Publications No. 61-10806 (1985), No. 63-100058 (1988) No. 60-19603(1985)).

Although the BaO--TiO₂ based materials have a dielectric constant εr of 37 to 40 and a high Q value on the order of 40,000, it is difficult to achieve a resonant frequency temperature coefficient τf of zero in a single phase (the resonant frequency temperature coefficient τf represents the temperature dependence of the dielectric constant). Further, these materials suffer from a great change in the dielectric constant and the temperature dependence of the dielectric constant with respect to a change in the composition thereof. Therefore, it is difficult to stably reduce the resonant frequency temperature coefficient τf while ensuring a high dielectric constant and a low dielectric loss.

Known as the BaO--REO--TiO₂ based materials are BaO--Nd₂ O₃ --TiO₂ based materials and BaO--Sm₂ O₃ --TiO₂ based materials. These materials have a dielectric constant εr of 40 to 60, and some of them have a resonant frequency temperature coefficient τf of zero. However, the materials have a low Q value on the order of 5,000 or lower.

The MgTiO₃ --CaTiO₃ based materials have a high Q value on the order of 30,000, and some of them have a resonant frequency temperature coefficient τf of zero. However, the materials have a low dielectric constant on the order of 16 to 25.

Therefore, none of the above materials perfectly satisfy the aforesaid three principal properties required of the dielectric material for high frequency applications.

SUMMARY OF THE INVENTION

It is a main object of the present invention to provide a dielectric ceramic composition which has a high dielectric constant, a high Q value and a stably low temperature dependence of the dielectric constant.

It is another object of the invention to provide a preparation method for the aforesaid dielectric ceramic composition, and to provide a dielectric resonator employing the dielectric ceramic composition.

As a result of intensive studies to solve the aforesaid problems, the inventor of the present invention has newly found that a dielectric ceramic composition containing at least Ca, Ti, La, Zn and M (which is at least one element selected from the group consisting of W, Ni, Co, Zr, Sn, Mn and Cu) as metal elements in a certain composition ratio has a high dielectric constant, a high Q value, a stably low temperature dependence of the dielectric constant in the high frequency region.

More specifically, there is provided a dielectric ceramic composition represented on a molar fraction basis by the following composition formula:

    aCaO·bTiO.sub.2 ·cLa.sub.2 O.sub.3 dZnO·eMO.sub.x

wherein M is as defined above 1≦x≦≦3; and a to e are molar fractions defined as 0.050≦a≦≦0.320, 0.150≦b≦0.470, 0.100≦c≦300, 0.100≦d≦0.300 and 0<e≦0.250, respectively (a+b+c+d+e=1).

In the aforesaid formula, the molar fractions a, b, c, d and e preferably satisfy the following expression:

    0.40≦{a+(c/2)}/{(b/2)+d+e}<1.00

Thus, the composition has an improved Q value.

The dielectric ceramic composition preferably contains plate crystals after having been formed into a predetermined shape and sintered. Thus, the composition has a further improved Q value.

A preferred example of the dielectric ceramic composition according to the present invention, in which the metal element M is tungsten W, is represented by the following composition formula:

    aCaO·bTiO.sub.2 ·cLa.sub.2 O.sub.3 ·dZnO·eWO.sub.3

(wherein a, b, c, d and e are as defined above), and contains plate crystals.

The plate crystals contain a greater amount of the metal element M, preferably tungsten W, than the other crystals in the composition, and are preferably present in a proportion of 0.02 to 10 vol % in the dielectric ceramic composition.

The dielectric ceramic composition is prepared by mixing oxides of the respective metal elements constituting the aforesaid composition formula, forming the resulting mixture into a predetermined shape, and firing the resulting formed product at a temperature of 1,400 to 1,650° C.

The dielectric ceramic composition containing the plate crystals is prepared by firing the formed product at a temperature of 1,400 to 1,650° C., and then reducing the firing temperature at a rate of 2 to 300° C. per hour.

In accordance with the present invention, there is also provided a dielectric resonator which comprises a resonator medium of a predetermined shape composed of the aforesaid dielectric ceramic composition, and a pair of input and output terminals disposed on opposite sides of the resonator medium for applying a high frequency signal to the resonator medium so as to resonate the resonator medium at a predetermined resonant frequency.

Other objects and advantages of the present invention will become apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic trace of an exemplary photograph illustrating crystals in a composition according to the present invention; and

FIG. 2 is a sectional diagram illustrating one embodiment of a dielectric resonator according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the composition formula aCaO·bTiO₂ ·cLa₂ O₃ ·dZnO·eMO_(x) representing the dielectric ceramic composition of the present invention, the molar fractions a, b, c, d and e are set within predetermined ranges. This is based on the following ground.

The molar fraction "a" is defined as 0.050≦a≦0.320. If a<0.050, the Q value and the resonant frequency temperature coefficient τf are reduced. If a>0.320, the Q value is reduced and the τf value is increased. Particularly, it is desirable that the molar fraction "a" is defined as 0.100≦a≦0.270.

The molar fraction "b" is defined as 0.150≦b≦0.470. If b<0.150, the Q value is reduced. Ifb>0.470, the Q value is reduced and the τf value is increased. Particularly, it is desirable that the molar fraction "b" is defined as 0.200≦b≦0.420.

The molar fraction "c" is defined as 0.100≦c≦0.300. If c<0.100, the Q value is reduced and the τf value is increased. If c>0.300, the Q value is reduced. Particularly, it is desirable that the molar fraction "c" is defined as 0.120≦c≦0.270.

The molar fraction "d" is defined as 0.100≦d≦0.300. If d<0.100, the Q value is reduced and the τf value is increased. If d>0.300, the Q value is reduced. Particularly, it is desirable that the molar fraction "d" is defined as 0.120≦d≦0.270.

The molar fraction "e" is defined as 0<e≦0.250. If e=0, the Q value is reduced and the τf value is increased. If e>0.250, the Q value is reduced. Particularly, the molar fraction "e" is desirably defined as 0.001≦e≦0.200, more desirably 0.003≦e≦0.200.

For improvement of the Q value, the molar fractions preferably satisfy the following expression:

    0.40≦{a+(c/2)}/{(b/2)+d+e}≦1.00,

particularly desirably the following expression:

    0.45≦{a+(c/2)}/{(b/2)+d+e}≦0.95

In the present invention, the metal element M to be contained in the composition is at least one element selected from the group consisting of W, Ni, Co, Zr, Sn, Mn and Cu. This is based on the following ground:

In general, CaTiLaZn based ceramic compositions tend to have a great positive τf value, so that it is difficult to control the τf value at about zero while ensuring a higher dielectric constant and a higher Q value. In the present invention, at least one element selected from W, Ni, Co, Zr, Sn, Mn and Cu is added to a CaTiLaZn based ceramic composition, whereby the τf value is successfully controlled at about zero while the dielectric constant and the Q value are maintained at high levels. It is particularly preferred to use W.

In the present invention, the dielectric ceramic composition preferably contains plate crystals, typically, in a proportion of 0.02 to 10 vol %. If the content of the plate crystals is lower than 0.02 vol % or greater than 10 vol %, the Q value may be reduced.

The dielectric ceramic composition containing the plate crystals is prepared by mixing oxides of the respective metal elements constituting the aforesaid formula in a predetermined ratio, forming the resulting mixture into a predetermined shape, firing the resulting formed product at a firing temperature of 1,400 to 1,650° C., and then reducing the firing temperature at a temperature reduction rate of 2 to 300° C. per hour.

The amount of the plate crystals to be formed in the composition can be controlled within the aforesaid range by controlling the temperature reduction rate at 2 to 300° C. per hour during the firing, whereby the Q value is improved. The firing temperature is desirably reduced at a temperature reduction rate of 2 to 300° C. per hour, more desirably 4 to 70° C. per hour, from the maximum firing temperature to 800° C. Thus, the Q value is further improved. If the temperature reduction rate is greater than 300° C. per hour, the plate crystals do not sufficiently grow, resulting in a lower Q value.

In the present invention, the plate crystals are defined as follows. When a surface of the dielectric ceramics which has been polished to planarity and mirror-finished is photographed by means of a wavelength dispersive X-ray microanalyzer at an acceleration voltage of 15 kV, a probe current of 5×10⁻¹⁰ A and a magnification of about 1,000 to 5,000 to obtain a reflective electron image photograph, crystals more whitish than the other crystals are observed in the photograph. These whitish crystals are planer and have various shapes such as T-, L-, trapezoidal and polygonal shapes. Some of the whitish crystals may have projections and indentations, curves, deformations, voids and the like. The crystals have dimensions of about 1 to 100 μm as measured along the longest axis, about 0.5 to 30 μm as measured along the second longest axis and about 0.1 to 10 μm as measured along the shortest axis. When the mirror-finished surface is observed by means of the wavelength dispersive X-ray microanalyzer by application of a characteristic X-ray at an acceleration voltage of 15 kV and a probe current of 1.0×10⁻⁷ A, it is found that the whitish crystals contain a greater amount of the metal element M (e.g., W) than the other crystals. Thus, the plate crystals are herein defined as crystals which look whitish in a reflective electron image photograph and contain a greater amount of the metal element M than the other crystals.

FIG. 1 is a trace of a reflective electron image photograph taken at a magnification of 2,000. In FIG. 1, whitish crystals indicated by a reference numeral 10 are the plate crystals.

The presence of the plate crystals and the presence of the metal M in the plate crystals are confirmed in the following manner. A surface of the dielectric ceramics is polished to planarity and mirror-finished, and the mirror-finished surface is photographed by means of a wavelength dispersive X-ray microanalyzer at an acceleration voltage of 15 kV, a probe current of 5×10⁻¹⁰ A and a magnification of about 1,000 to 5,000 to obtain a reflective electron image photograph. The presence of the plate crystals is confirmed by presence of more whitish portions than the other portions in the photograph. The presence of the metal M (particularly, W) in the plate crystals is determined by application of a characteristic X-ray at an acceleration voltage of 15 kV and a probe current of 1.0×10⁻⁷ A.

The proportion of the plate crystals is determined on the basis of volume percentage by determining the area ratio of plate crystal portions in a reflective electron image photograph. The reflective electron image photograph is taken by means of a wavelength dispersive X-ray microanalyzer at an acceleration voltage of 15 kV, a probe current of 5×10⁻¹⁰ A and a magnification of about 1,000 to 5,000. The area ratio of the plate crystal portions 10 in the photograph shown in FIG. 1 is determined by an image analysis (e.g. tradename "LUZEX" manufactured by NIRECO Co., or the like) or a photograph tracing method. Where the area ratio of the plate crystals is determined through the image analysis, for example, it is necessary to trace only the plate crystal portions in the photograph on a paper sheet prior to the image analysis for elimination of influences of voids and grain omission.

The determination of the area ratio of the plate crystals by the photograph tracing method is achieved in the following manner. The plate crystal portions and the other portions in the photograph are separately traced on a homogeneous paper sheet, for example, a cross-ruled sheet, and the area ratio of the plate crystal portions in the trace is determined on the basis of the grid scale. Alternatively, the plate crystal portions in the trace on the paper sheet are cut out and weighed, and the ratio of the total weight of the cut-out plate crystal portions to the weight of the other trace portions is determined as the area ratio of the p late crystals.

Since a sintered product having a smaller number of oxygen vacancies generally has an improved Q value, the following is supposedly the reason why the presence of the plate crystals improves the Q value in the invention.

The amount of the plate crystals formed in the sintered product and the Q value differ depending on the temperature reduction rate in the firing process even with the same content of the metal element M. Therefore, it is considered that the temperature reduction rate is correlated with the amount of the formed plate crystals and the Q value. More specifically, the plate crystals containing the metal element M are properly formed in the sintered product by controlling the temperature reduction rate within a certain range, whereby the oxygen vacancies in the sintered product are reduced to improve the Q value. If the temperature reduction rate is too high, the formation of the plate crystals may be insufficient, resulting in a greater number of oxygen vacancies and hence a lower Q value. If the temperature reduction rate is too low, conversely, the formation of the plate crystals may be undesirably increased to greater than the aforesaid range, and the composition may be changed due to evaporation and reactions with firing jigs, whereby the Q value is reduced. In addition, the time required for the firing process is prolonged, so that mass production is impossible. if the amount of the formed plate crystals is greater than the aforesaid range, the sintered product may have a lower Q value because the Q value of the plate crystals is low.

In the present invention, the amount of the plate crystals to be formed in the dielectric ceramic composition is controlled within the aforesaid range by controlling the temperature reduction rate at 2 to 300° C. per hour, whereby the Q value is improved.

The dielectric ceramic composition of the present invention can be prepared, for example, in the following manner. Highly pure powdery calcium carbonate (CaCO₃) titanium oxide (TiO₂), lanthanum oxide (La₂ O₃), zinc oxide (ZnO), and the oxide of the metal element M [at least one oxide selected from tungsten oxide (WO₃) nickel oxide (NiO), cobalt oxide (CoO), zirconium oxide (ZrO₂), tin oxide (SnO₂), manganese carbonate (MnCO₃) and copper oxide (CuO)] are used as starting materials. These powdery starting materials are weighed and blended in a predetermined ratio, and pure water is added to the resulting blend, which is in turn mixed and milled in wet process for about 10 to 30 hours until the average particle diameter of the blended materials is reduced to 2.0 m or smaller. A ball milling which employs zirconia balls, for example, is used for the wet mixing and milling process.

The resulting mixture of the powdery materials is dried, and pre-fixed at 1,000 to 1,300° C. for 2 to 10 hours. Then, 5 wt % of a binder is added to the pre-fired mixture, which is in turn granulated. The resulting powder is formed into a given shape by known forming means, for example, by mold press, cold hydrostatic press, extrusion or the like, and then the formed product is fired in an air atmosphere at a temperature of 1,400 to 1,650 C for 1 to 100 hours. After the firing, the firing temperature is preferably reduced at the predetermined temperature reduction rate as described above.

It is noted that impurities such as SiO₂, Al₂ O₃ and other rare earth elements may be contained in an amount of not greater than 1 wt % in the resulting product, depending on the kinds of the starting materials to be used, the in of the milling balls and other preparation conditions.

The dielectric resonator according to the present invention, which may be of a TE mode type as shown in FIG. 2, comprises a metallic case 1, an input terminal 2 and an output terminal 3 provided on opposed interior surfaces of the metal case 1, and a dielectric ceramic body 4 composed of the aforesaid dielectric ceramic composition as a resonator medium and disposed between the input and output terminals 2 and 3. In such a TE mode dielectric resonator, microwaves inputted through the input terminal 2 are reflected on an interface between the dielectric ceramic body 4 and a free space thereby to be confined within the dielectric ceramic body 4, which in turn resonates at a predetermined frequency. A signal generated in this manner is electromagnetically coupled to the output terminal 3 thereby to be outputted therethrough.

Needless to say, the dielectric ceramic composition of the present invention is applicable to TEM-mode coaxial resonators and strip line resonators, TM-mode dielectric ceramic resonators, and other resonators. Further, the dielectric resonator may be constructed such that the input terminal 2 and the output terminal 3 are provided directly on the dielectric ceramic body 4.

The dielectric ceramic body 4, which is a resonator medium composed of the dielectric ceramic composition of the invention, has a predetermined shape, for example, rectangular solid, cube, disk, cylinder, polygonal column or a like solid which allows for resonance of the resonator body. It is preferred in practice that the frequency of the high frequency signal to be inputted is about 200 MHz to about 300 GHz and the resonant frequency is about 300 MHz to about 100 GHz.

The dielectric resonator of the invention can advantageously be applied to resonators, MIC dielectric substrates, dielectric waveguides, dielectric antennas and any of various electronic components which are operative in microwave and millimeter-wave regions, for example, for car telephones, cordless telephones, personal wireless units, satellite broadcast receivers and the like.

EXAMPLES

The dielectric ceramic composition of the present invention will hereinafter be described by way of examples thereof. It should be understood that the invention be not limited to these examples.

Example 1

Highly pure calcium carbonate (CaCO₃), titanium oxide (TiO₂), lanthanum oxide (La₂ O₃) and zinc oxide (ZnO) were used as essential starting materials. Powdery tungsten oxide (WO₃), nickel oxide (NiO), cobalt oxide (CoO), zirconium oxide (ZrO₂), tin oxide (SnO₂) manganese carbonate (MnCO₃) and copper oxide (CuO) were each used as another starting material MO_(x) (wherein M and x are as defined above) to be mixed with the essential starting materials.

These starting materials were weighed and blended in molar ratios listed in Tables 1 and 2, and pure water was added to the resulting blend, which was in turn mixed and milled in wet process for about 20 hours by means of a ball mill until the average particle diameter of the blended materials was reduced to 2.0 μm or smaller. It is noted that the molar ratios of CaCO₃ and MnCO₃ correspond to the molar fractions of CaO and MnO₂, respectively, shown in the tables, and a molar fraction e in Table 2 is the total of molar fractions 1, 2 and 3.

The mixture was dried, and pre-fired at 1,150° C. for 2 hours. The n, 5 wt % of a binder was added to the pre-fired mixture, which was in turn granulated. The resulting powder was press-molded at a pressure of about 1 ton/cm² into a cylindrical form, which was in turn fired in an air atmosphere at a temperature of 1,450 to 1,650° C. for 2 hours.

A flat surface of the resulting ceramic product was polished to planarity, and ultrasonically washed in acetone. After the ceramic product is dried at 15020 C. for one hour, the dielectric constant εr, Q value and resonant frequency temperature coefficient τf of the ceramic product were measured at a measurement frequency of 3 to 4 GHz by a cylinder resonator method.

The Q value thus obtained was converted to a Q value at 1 GHz on the basis of the following relation:

    Q value×Measurement frequency f=Constant

which has been generally established for microwave dielectric materials. The resonant frequency temperature coefficient τf was measured in a temperature range of 20 to 85° C. The results are also shown in Tables 1 and 2.

As apparent from Tables 1 and 2, dielectric ceramic products of comparative examples each had a lower dielectric constant and/or a lower Q value, or a τf value of lower than -30 ppm/° C. or greater than 50 ppm/° C.

On the contrary, dielectric ceramic products obtained in accordance with the present invention each had excellent dielectric characteristics, i.e., a dielectric constant of not lower than 45, a Q value of not lower than 20,000 at 1 GHz and a τf value of -30 to +50 ppm/° C. With substantially the same molar fractions a, b, c and d, dielectric ceramic products having a greater content (molar fraction e) of MO_(x) (wherein M and x are as defined above) had a higher dielectric constant and a higher Q value, and a τf value of virtually zero. This is evident from a comparison among Samples No. 15, No. 16 and No. 17, a comparison among samples No. 47, No. 48 and No. 49, and the like.

It is noted that the molar fractions of the respective metal oxides in each ceramic product sample shown in Tables 1 and 2 were the same as those in the starting formulation.

                                      TABLE 1                                      __________________________________________________________________________                               Dielectric                                           Sample                                                                             a  b  c   d  e  (a + c/2)/                                                                           Constant τf                                      No. CaO                                                                               TiO.sub.2                                                                         La.sub.2 O.sub.3                                                                   ZnO                                                                               WO.sub.3                                                                          (b/2 + d + e)                                                                        εr                                                                          Q value                                                                            (ppm/° C.)                           __________________________________________________________________________     1   0.320                                                                             0.380                                                                             0.100                                                                              0.100                                                                             0.100                                                                             0.949 66.3 21000                                                                              49.0                                        2   0.270                                                                             0.240                                                                             0.150                                                                              0.170                                                                             0.170                                                                             0.750 50.5 36000                                                                              -11.7                                       3   0.100                                                                             0.370                                                                             0.260                                                                              0.260                                                                             0.010                                                                             0.505 48.2 39000                                                                              -23.8                                       4   0.050                                                                             0.360                                                                             0.300                                                                              0.260                                                                             0.030                                                                             0.426 45.0 22000                                                                              -30.0                                       5   0.280                                                                             0.470                                                                             0.110                                                                              0.110                                                                             0.030                                                                             0.893 65.1 20000                                                                              50.0                                        6   0.220                                                                             0.420                                                                             0.170                                                                              0.170                                                                             0.020                                                                             0.763 63.5 35000                                                                              11.3                                        7   0.240                                                                             0.200                                                                             0.210                                                                              0.200                                                                             0.150                                                                             0.767 48.0 31000                                                                              -19.9                                       8   0.140                                                                             0.150                                                                             0.270                                                                              0.270                                                                             0.170                                                                             0.534 46.7 24000                                                                              -28.1                                       9   0.200                                                                             0.250                                                                             0.300                                                                              0.200                                                                             0.050                                                                             0.933 45.8 22000                                                                              -27.3                                       10  0.100                                                                             0.370                                                                             0.270                                                                              0.240                                                                             0.020                                                                             0.528 45.6 38000                                                                              -9.6                                        11  0.270                                                                             0.390                                                                             0.120                                                                              0.190                                                                             0.030                                                                             0.795 52.2 36000                                                                              0.0                                         12  0.250                                                                             0.270                                                                             0.100                                                                              0.180                                                                             0.200                                                                             0.583 46.9 20000                                                                              -25.2                                       13  0.080                                                                             0.350                                                                             0.267                                                                              0.300                                                                             0.003                                                                             0.447 45.0 23000                                                                              -29.4                                       14  0.200                                                                             0.200                                                                             0.250                                                                              0.270                                                                             0.080                                                                             0.722 54.4 39000                                                                              17.5                                        15  0.293                                                                             0.420                                                                             0.135                                                                              0.147                                                                             0.005                                                                             0.996 53.0 35000                                                                              41.8                                        16  0.282                                                                             0.412                                                                             0.130                                                                              0.126                                                                             0.050                                                                             0.908 53.7 36000                                                                              20.5                                        17  0.250                                                                             0.385                                                                             0.115                                                                              0.110                                                                             0.140                                                                             0.695 53.5 37000                                                                              0.0                                         18  0.250                                                                             0.300                                                                             0.200                                                                              0.100                                                                             0.150                                                                             0.875 46.8 26000                                                                              -28.7                                       19  0.080                                                                             0.150                                                                             0.290                                                                              0.230                                                                             0.250                                                                             0.405 48.9 20000                                                                              -30.0                                       20  0.225                                                                             0.225                                                                             0.175                                                                              0.175                                                                             0.200                                                                             0.641 45.1 39000                                                                              0.0                                         21  0.260                                                                             0.380                                                                             0.130                                                                              0.227                                                                             0.003                                                                             0.774 51.7 40000                                                                              -5.9                                        22  0.215                                                                             0.400                                                                             0.190                                                                              0.194                                                                             0.001                                                                             0.785 55.7 35000                                                                              10.3                                        *23 0.380                                                                             0.350                                                                             0.100                                                                              0.100                                                                             0.070                                                                             1.246 83.2  3000                                                                              287.3                                       *24 0.010                                                                             0.150                                                                             0.300                                                                              0.300                                                                             0.240                                                                             0.260 Unmeasurable                                         *25 0.310                                                                             0.520                                                                             0.080                                                                              0.080                                                                             0.010                                                                             1.000 132.5                                                                               1000                                                                               360.2                                       *26 0.320                                                                             0.100                                                                             0.200                                                                              0.200                                                                             0.180                                                                             0.977 Unmeasurable                                         *27 0.200                                                                             0.150                                                                             0.350                                                                              0.250                                                                             0.050                                                                             1.000 29.7 2000                                                                               -49.4                                       *28 0.200                                                                             0.450                                                                             0.050                                                                              0.200                                                                             0.100                                                                             0.429 142.0                                                                               1000                                                                               585.8                                       *29 0.100                                                                             0.300                                                                             0.200                                                                              0.350                                                                             0.050                                                                             0.364 26.5 4000                                                                               -47.2                                       *30 0.320                                                                             0.350                                                                             0.200                                                                              0.050                                                                             0.080                                                                             1.377 122.8                                                                               6000                                                                               316.9                                       *31 0.100                                                                             0.200                                                                             0.300                                                                              0.100                                                                             0.300                                                                             0.500 Unmeasurable                                         *32 0.250                                                                             0.300                                                                             0.250                                                                              0.200                                                                             0.000                                                                             1.071 47.0 6000                                                                               66.1                                        __________________________________________________________________________      Sample Numbers prefixed with * are comparative examples.                 

                                      TABLE 2                                      __________________________________________________________________________     Sam-                                               Dielectric                                                                              τf             ple                                                                               a  b  c   d  e MO.sub.x                   (a + c/2)/                                                                           Constant (ppm/              No.                                                                               CaO                                                                               TiO.sub.2                                                                         La.sub.2 O.sub.3                                                                   ZnO                                                                               1      2      3      1 + 2 + 3                                                                              (b/2 + d + e)                                                                        εr                                                                          Q                                                                                  °           __________________________________________________________________________                                                                 C.)                33 0.320                                                                             0.380                                                                             0.100                                                                              0.100                                                                             NiO 0.100                                                                             --  -- --  -- 0.100   0.949 67.7 21000                                                                              49.5               34 0.270                                                                             0.240                                                                             0.150                                                                              0.170                                                                             CoO 0.170                                                                             --  -- --  -- 0.170   0.750 53.9 33000                                                                              -5.7               35 0.100                                                                             0.370                                                                             0.260                                                                              0.260                                                                             ZrO.sub.2                                                                          0.010                                                                             --  -- --  -- 0.010   0.505 51.4 33000                                                                              -16.1              36 0.050                                                                             0.360                                                                             0.300                                                                              0.260                                                                             SnO.sub.2                                                                          0.030                                                                             --  -- --  -- 0.030   0.426 46.2 22000                                                                              -29.5              37 0.280                                                                             0.070                                                                             0.110                                                                              0.110                                                                             MnO.sub.2                                                                          0.030                                                                             --  -- --  -- 0.030   0.893 66.7 21000                                                                              48.7               38 0.220                                                                             0.420                                                                             0.170                                                                              0.170                                                                             CuO 0.020                                                                             --  -- --  -- 0.020   0.763 60.6 30000                                                                              22                 39 0.240                                                                             0.200                                                                             0.210                                                                              0.200                                                                             NiO 0.140                                                                             CoO 0.010                                                                             --  -- 0.150   0.767 46.9 29000                                                                              -19.9              40 0.140                                                                             0.150                                                                             0.270                                                                              0.270                                                                             NiO 0.100                                                                             ZrO.sub.2                                                                          0.070                                                                             --  -- 0.170   0.534 48.1 26000                                                                              -26.3              41 0.200                                                                             0.260                                                                             0.300                                                                              0.200                                                                             NiO 0.010                                                                             SnO.sub.2                                                                          0.040                                                                             --  -- 0.050   0.921 46.4 21000                                                                              -24.0              42 0.100                                                                             0.370                                                                             0.270                                                                              0.240                                                                             NiO 0.010                                                                             MnO.sub.2                                                                          0.010                                                                             --  -- 0.020   0.528 47.0 39000                                                                              -7.3               43 0.270                                                                             0.390                                                                             0.120                                                                              0.190                                                                             NiO 0.010                                                                             CuO 0.020                                                                             --  -- 0.030   0.795 50.5 35000                                                                              0.1                44 0.250                                                                             0.270                                                                             0.100                                                                              0.180                                                                             NiO 0.190                                                                             WO.sub.2                                                                           0.010                                                                             --  -- 0.200   0.583 45.2 20000                                                                              -23.5              45 0.080                                                                             0.350                                                                             0.267                                                                              0.300                                                                             CoO 0.002                                                                             ZrO.sub.2                                                                          0.001                                                                             --  -- 0.003   0.447 46.2 23000                                                                              -29.1              46 0.200                                                                             0.200                                                                             0.250                                                                              0.270                                                                             CoO 0.020                                                                             SnO.sub.2                                                                          0.060                                                                             --  -- 0.080   0.722 55.7 38000                                                                              18.9               47 0.293                                                                             0.420                                                                             0.135                                                                              0.147                                                                             CoO 0.001                                                                             MnO.sub.2                                                                          0.004                                                                             --  -- 0.005   0.996 54.0 34000                                                                              40.2               48 0.282                                                                             0.412                                                                             0.130                                                                              0.126                                                                             CoO 0.040                                                                             CoO 0.010                                                                             --  -- 0.050   0.908 54.5 36000                                                                              22.5               49 0.250                                                                             0.385                                                                             0.115                                                                              0.110                                                                             CoO 0.130                                                                             Wo.sub.3                                                                           0.010                                                                             --  -- 0.140   0.695 54.2 36000                                                                              0.0                50 0.250                                                                             0.300                                                                             0.200                                                                              0.100                                                                             ZrO.sub.2                                                                          0.070                                                                             SnO.sub.2                                                                          0.080                                                                             --  -- 0.150   0.875 48.2 25000                                                                              -26.3              51 0.080                                                                             0.150                                                                             0.290                                                                              0.230                                                                             ZrO.sub.2                                                                          0.240                                                                             MnO.sub.2                                                                          0.010                                                                             --  -- 0.250   0.405 50.2 21000                                                                              -29.3              52 0.225                                                                             0.225                                                                             0.175                                                                              0.175                                                                             ZrO.sub.2                                                                          0.150                                                                             CuO 0.050                                                                             --  -- 0.200   0.641 46.6 38000                                                                              0.4                53 0.260                                                                             0.380                                                                             0.130                                                                              0.227                                                                             ZrO.sub.2                                                                          0.001                                                                             WO.sub.3                                                                           0.002                                                                             --  -- 0.003   0.774 51.3 40000                                                                              -5.3               54 0.215                                                                             0.400                                                                             0.189                                                                              0.194                                                                             SnO.sub.2                                                                          0.001                                                                             MnO.sub.2                                                                          0.001                                                                             --  -- 0.002   0.782 55.3 36000                                                                              11.3               55 0.320                                                                             0.380                                                                             0.100                                                                              0.100                                                                             SnO.sub.2                                                                          0.050                                                                             CuO 0.050                                                                             --  -- 0.100   0.949 46.8 37000                                                                              0.3                56 0.270                                                                             0.240                                                                             0.150                                                                              0.170                                                                             SnO.sub.2                                                                          0.160                                                                             MnO.sub.                                                                           0.010                                                                             --  -- 0.170   0.750 65.2 21000                                                                              48.2               57 0.100                                                                             0.370                                                                             0.260                                                                              0.260                                                                             NiO 0.003                                                                             CoO 0.003                                                                             ZrO.sub.2                                                                          0.004                                                                             0.010   0.505 51.3 35000                                                                              -10.3              58 0.050                                                                             0.360                                                                             0.300                                                                              0.260                                                                             NiO 0.010                                                                             CoO 0.010                                                                             SnO.sub.2                                                                          0.010                                                                             0.030   0.426 47.4 38000                                                                              -22.4              59 0.280                                                                             0.470                                                                             0.110                                                                              0.110                                                                             NiO 0.005                                                                             CoO 0.015                                                                             MnO.sub.2                                                                          0.010                                                                             0.030   0.893 48.3 22000                                                                              -28.5              60 0.220                                                                             0.420                                                                             0.170                                                                              0.170                                                                             NiO 0.010                                                                             CoO 0.005                                                                             CuO 0.005                                                                             0.020   0.763 65.8 21000                                                                              47.6               61 0.240                                                                             0.200                                                                             0.210                                                                              0.200                                                                             NiO 0.010                                                                             CoO 0.020                                                                             WO.sub.3                                                                           0.120                                                                             0.150   0.767 66.0 32000                                                                              13.5               62 0.140                                                                             0.150                                                                             0.270                                                                              0.270                                                                             CoO 0.160                                                                             ZrO.sub.2                                                                          0.005                                                                             SnO.sub.2                                                                          0.005                                                                             0.170   0.534 48.2 35000                                                                              -5.2               63 0.200                                                                             0.250                                                                             0.300                                                                              0.200                                                                             CoO 0.010                                                                             SnO.sub.2                                                                          0.010                                                                             MnO.sub.2                                                                          0.030                                                                             0.050   0.933 49.7 25000                                                                              -29.1              64 0.100                                                                             0.370                                                                             0.270                                                                              0.240                                                                             CoO 0.010                                                                             MnO.sub.2                                                                          0.005                                                                             CuO 0.005                                                                             0.020   0.528 45.9 29000                                                                              -25.9              65 0.270                                                                             0.390                                                                             0.120                                                                              0.190                                                                             CoO 0.015                                                                             CuO 0.005                                                                             WO.sub.3                                                                           0.010                                                                             0.030   0.795 46.9 37000                                                                              -10.5              66 0.250                                                                             0.270                                                                             0.100                                                                              0.180                                                                             ZrO.sub.2                                                                          0.095                                                                             SnO.sub.2                                                                          0.100                                                                             MnO.sub.2                                                                          0.005                                                                             0.200   0.583 53.7 34000                                                                              0.1                67 0.080                                                                             0.350                                                                             0.267                                                                              0.300                                                                             ZrO.sub.2                                                                          0.001                                                                             SnO.sub.2                                                                          0.001                                                                             WO.sub.3                                                                           0.001                                                                             0.003   0.447 47.8 21000                                                                              -26.3              68 0.200                                                                             0.200                                                                             0.250                                                                              0.270                                                                             SnO.sub.2                                                                          0.060                                                                             MnO.sub.2                                                                          0.010                                                                             CuO 0.010                                                                             0.080   0.722 46.4 21000                                                                              -29.8              69 0.293                                                                             0.420                                                                             0.135                                                                              0.147                                                                             SnO.sub.                                                                           0.002                                                                             MnO.sub.2                                                                          0.001                                                                             WO.sub.3                                                                           0.002                                                                             0.005   0.996 5.71 36000                                                                              17.6               *70                                                                               0.380                                                                             0.350                                                                             0.100                                                                              0.100                                                                             NiO 0.070                                                                             --  -- --  -- 0.070   1.246 85.2  2000                                                                              331.8              *71                                                                               0.010                                                                             0.150                                                                             0.300                                                                              0.300                                                                             CoO 0.240                                                                             --  -- --  -- 0.240   0.260 Unmeasurable                *72                                                                               0.310                                                                             0.520                                                                             0.080                                                                              0.080                                                                             ZrO.sub.2                                                                          0.010                                                                             --  -- --  -- 0.010   1.000 157.5                                                                                1000                                                                              326.7              *73                                                                               0.320                                                                             0.100                                                                             0.200                                                                              0.200                                                                             SnO.sub.2                                                                          0.180                                                                             --  -- --  -- 0.180   0.977 Unmeasurable                *74                                                                               0.200                                                                             0.150                                                                             0.350                                                                              0.250                                                                             MnO.sub.2                                                                          0.050                                                                             --  -- --  -- 0.050   1.000 38.0  1000                                                                              -42.4              *75                                                                               0.200                                                                             0.450                                                                             0.050                                                                              0.200                                                                             CuO 0.100                                                                             --  -- --  -- 0.100   0.429 Unmeasurable                *76                                                                               0.100                                                                             0.300                                                                             0.200                                                                              0.350                                                                             NiO 0.040                                                                             CoO 0.010                                                                             --  -- 0.050   0.364 34.6  4000                                                                              -43.0              *77                                                                               0.320                                                                             0.350                                                                             0.200                                                                              0.050                                                                             CoO 0.060                                                                             SnO.sub.2                                                                          0.010                                                                             WO.sub.3                                                                           0.010                                                                             0.080   1.377 136.9                                                                                7000                                                                              427.4              *78                                                                               0.100                                                                             0.200                                                                             0.300                                                                              0.100                                                                             ZrO.sub.2                                                                          0.200                                                                             CuO 0.100                                                                             --  -- 0.300   0.500 Unmeasurable                __________________________________________________________________________      Sample Numbers prefixed with * are comparative examples.                 

Example 2

Highly pure powdery calcium carbonate (CaCO₃), titanium oxide (TiO₂), lanthanum oxide (La₂ O₃), zinc oxide (ZnO) and tungsten oxide (WO₃) were used as starting materials. These starting materials were weighed and blended in molar ratios listed in Table 3, and pure water was added to the resulting blend, which was in turn mixed and milled in wet process for about 20 hours by means of a ball mill until the average particle diameter of the blended materials was reduced to 2.0μm or smaller.

The resulting mixture was dried, and pre-fired at 1,150° C. for 2 hours. Then, 5 wt % of a binder was added to the pre-fired mixture, which was in turn granulated. The resulting powder was press-molded at a pressure of about 1 ton/cm² into a cylindrical form, which was in turn fired in an air atmosphere at a temperature of 1,450 to 1,650° C. for 2 hours, and then the firing temperature was reduced at a temperature reduction rate of 2 to 250° C. per hour down to 800° C.

The dielectric constant τr, Q value and resonant frequency temperature coefficient τf of the resulting dielectric ceramic product were measured in the same manner as in Example 1. The results are shown in Table 3.

Further, a surface of the ceramic product was polished to planarity and mirror-finished, and then the mirror-finished surface was observed in the following manner by means of a wavelength dispersive X-ray microanalyzer for determination of the presence of plate crystals in the ceramic product, the presence of W in the plate crystals and the volume percentage of the plate crystals. The volume percentage of the plate crystals in each ceramic product sample is shown in Table 3.

The presence of the plate crystals in the ceramic product was determined on the basis of reflective electron image photographs respectively taken at an acceleration voltage of 15 kV, a probe current of 5×10⁻¹⁰ A and magnifications of 1,000, 2,000 and 5,000. The plate crystal portions looked more whitish than the other portions in the photographs. The whitish plate crystal portions in the photograph taken at a magnification of 2,000 were traced as black solid portions on a paper sheet, and the area ratio of the plate crystal portions to the other portions was determined by an image analysis (tradename "LUZEX"). The area ratio of the plate crystals thus determined was defined as the volume percentage of the plate crystals in the ceramic product.

The presence of W in the plate crystals was determined by application of a characteristic X-ray at an acceleration voltage of 15 kV and a probe current of 1.0×10⁻⁷ A. As a result, W was contained in plate crystals in all the samples No. 79 to No. 105. Further, it was found that the plate crystals contained a greater amount of W than the other crystals.

As apparent from Table 3, dielectric ceramic products of comparative examples each had a lower dielectric constant and/or a lower Q value, or a r f value of lower than -30 ppm/° C. or greater than 50 ppm/° C.

On the contrary, dielectric ceramic products obtained in accordance with the present invention each had excellent dielectric characteristics, i.e., a dielectric constant of not lower than 45, a Q value of not lower than 20,000 at 1 GHz and a τf value of -30 to +50 ppm/° C.

It is noted that the molar fractions of the respective metal oxides in each ceramic product sample shown in Table 3 were the same as those in the starting formulation.

                                      TABLE 3                                      __________________________________________________________________________                               Temperature                                                                    reduction                                                                            Plate                                                                              Dielectric                                 Sample                                                                             a  b  c   d  e  (a + c/2)/                                                                           rate in firing                                                                       crystals                                                                           Constant τf                            No. CaO                                                                               TiO.sub.2                                                                         La.sub.2 O.sub.3                                                                   ZnO                                                                               WO.sub.3                                                                          (b/2 + d + e)                                                                        (° C./hour)                                                                   (Vol %)                                                                            εr                                                                          Q value                                                                            (ppm/° C.)                 __________________________________________________________________________     79  0.320                                                                             0.381                                                                             0.101                                                                              0.101                                                                             0.097                                                                             0.954 10    5.12                                                                               66.3 21000                                                                              49.0                              80  0.270                                                                             0.235                                                                             0.155                                                                              0.170                                                                             0.170                                                                             0.760 4     6.27                                                                               50.5 36000                                                                              -11.7                             81  0.270                                                                             0.235                                                                             0.155                                                                              0.170                                                                             0.170                                                                             0.760 300   8.37                                                                               48.3 29000                                                                              -10.0                             82  0.100                                                                             0.365                                                                             0.265                                                                              0.260                                                                             0.010                                                                             0.514 4     1.21                                                                               48.2 39000                                                                              -23.8                             83  0.100                                                                             0.365                                                                             0.265                                                                              0.260                                                                             0.010                                                                             0.514 50    1.03                                                                               47.0 33000                                                                              -22.5                             84  0.050                                                                             0.360                                                                             0.300                                                                              0.260                                                                             0.030                                                                             0.426 100   2.44                                                                               45.0 22000                                                                              -30.0                             85  0.285                                                                             0.470                                                                             0.105                                                                              0.110                                                                             0.030                                                                             0.900 200   0.37                                                                               65.1 20000                                                                              50.0                              86  0.230                                                                             0.420                                                                             0.170                                                                              0.160                                                                             0.020                                                                             0.808 40    0.09                                                                               63.5 35000                                                                              11.3                              87  0.230                                                                             0.420                                                                             0.170                                                                              0.160                                                                             0.020                                                                             0.808 100   0.12                                                                               63.2 32000                                                                              11.1                              88  0.230                                                                             0.210                                                                             0.210                                                                              0.200                                                                             0.150                                                                             0.736 2     4.84                                                                               48.0 31000                                                                              -19.9                             89  0.230                                                                             0.210                                                                             0.210                                                                              0.200                                                                             0.150                                                                             0.736 250   3.89                                                                               46.8 22000                                                                              -18.5                             90  0.150                                                                             0.150                                                                             0.260                                                                              0.270                                                                             0.170                                                                             0.544 20    3.94                                                                               46.7 24000                                                                              -28.1                             91  0.190                                                                             0.260                                                                             0.300                                                                              0.200                                                                             0.050                                                                             0.895 20    0.05                                                                               45.8 22000                                                                              -27.3                             92  0.100                                                                             0.360                                                                             0.280                                                                              0.240                                                                             0.020                                                                             0.545 10    0.15                                                                               45.8 38000                                                                              -9.6                              93  0.270                                                                             0.400                                                                             0.120                                                                              0.180                                                                             0.030                                                                             0.805 70    0.63                                                                               52.2 36000                                                                              0.0                               94  0.270                                                                             0.400                                                                             0.120                                                                              0.180                                                                             0.030                                                                             0.805 300   0.36                                                                               52.5 30000                                                                              0.4                               95  0.260                                                                             0.260                                                                             0.100                                                                              0.180                                                                             0.200                                                                             0.608 2     7.31                                                                               46.9 20000                                                                              -25.2                             96  0.080                                                                             0.340                                                                             0.277                                                                              0.300                                                                             0.003                                                                             0.462 20    0.06                                                                               45.0 23000                                                                              -29.4                             97  0.190                                                                             0.200                                                                             0.260                                                                              0.270                                                                             0.080                                                                             0.711 10    0.38                                                                               54.4 39000                                                                              17.5                              98  0.190                                                                             0.200                                                                             0.260                                                                              0.270                                                                             0.080                                                                             0.711 250   0.42                                                                               53.9 28000                                                                              16.6                              99  0.260                                                                             0.380                                                                             0.105                                                                              0.110                                                                             0.145                                                                             0.702 200   2.80                                                                               53.5 37000                                                                              0.0                               100 0.250                                                                             0.300                                                                             0.205                                                                              0.100                                                                             0.145                                                                             0.892 5     3.74                                                                               46.8 26000                                                                              -28.7                             101 0.080                                                                             0.150                                                                             0.295                                                                              0.225                                                                             0.250                                                                             0.414 20    9.58                                                                               48.9 20000                                                                              -30.0                             102 0.220                                                                             0.225                                                                             0.180                                                                              0.175                                                                             0.200                                                                             0.636 60    10.00                                                                              45.1 39000                                                                              0.0                               103 0.220                                                                             0.225                                                                             0.180                                                                              0.175                                                                             0200                                                                              0.636 250   8.82                                                                               45.1 35000                                                                              -0.3                              104 0.250                                                                             0.380                                                                             0.130                                                                              0.237                                                                             0.003                                                                             0.733 70    0.06                                                                               51.7 40000                                                                              -59                               105 0.210                                                                             0.400                                                                             0.190                                                                              0.199                                                                             0.001                                                                             0.763 50    0.02                                                                               55.7 35000                                                                              10.3                              *106                                                                               0.380                                                                             0.350                                                                             0.100                                                                              0.100                                                                             0.070                                                                             1.246 50    2.01                                                                               80.4 2000                                                                               283.5                             *107                                                                               0.010                                                                             0.150                                                                             0.300                                                                              0.300                                                                             0.240                                                                             0.260 50    15.83                                                                              Unmeasurable                               *108                                                                               0.310                                                                             0.520                                                                             0.080                                                                              0.080                                                                             0.010                                                                             1.000 50    0.01                                                                               125.6                                                                               2000                                                                               380.2                             *109                                                                               0.320                                                                             0.100                                                                             0.200                                                                              0.200                                                                             0.180                                                                             0.977 50    12.77                                                                              Unmeasurable                               *110                                                                               0.200                                                                             0.150                                                                             0.350                                                                              0.250                                                                             0.050                                                                             1.000 50    1.58                                                                               30.5 3000                                                                               -48.1                             *111                                                                               0.200                                                                             0.450                                                                             0.050                                                                              0.200                                                                             0.100                                                                             0.429 50    4.71                                                                               150.5                                                                               1000                                                                               593.0                             *112                                                                               0.100                                                                             0.300                                                                             0.200                                                                              0.350                                                                             0.050                                                                             0.364 50    009 24.5 3000                                                                               -47.1                             *113                                                                               0.320                                                                             0.350                                                                             0.200                                                                              0.050                                                                             0.080                                                                             1.377 50    2.69                                                                               125.6                                                                               5000                                                                               315.6                             *114                                                                               0.100                                                                             0.200                                                                             0.300                                                                              0.100                                                                             0.300                                                                             0.500 50    20.43                                                                              Unmeasurable                               *115                                                                               0.250                                                                             0.300                                                                             0.250                                                                              0.200                                                                             0.000                                                                             1.071 50    0.00                                                                               49.3 5000                                                                               67.4                              __________________________________________________________________________      Sample Numbers prefixed with * are comparative examples.                  

What is claimed is:
 1. A dielectric ceramic composition containing metal elements Ca, Ti, La, Zn and M which is at least one metal element selected from the group consisting of W, Ni, Co, Zr, Sn, Mn and Cu, and represented on a molar fraction basis by the following composition formula:

    aCaO·bTiO.sub.2 ·cLa.sub.2 O.sub.3 ·dZnO·eMO.sub.x

wherein M is defined above; 1≦x≦3; and a to e are molar fractions which are defined as 0.050≦a≦0.320, 0.150 ≦b≦0.470, 0.100≦c≦0.300, 0.100≦d≦0.300 and 0<e≦0.250, respectively (a+b+c+d+e=1).
 2. A dielectric ceramic composition as set forth in claim 1, wherein the molar fractions a, b, c, d and e satisfy the following expression:

    0.40≦{a+(c/2)}/{(b/2)+d+e}≦1.00.


3. A dielectric ceramic composition as set forth in claim 1, which contains plate crystals.
 4. A dielectric ceramic composition as set forth in claim 3, wherein the plate crystals are present in a proportion of 0.02 to 10 vol %.
 5. A dielectric ceramic composition as set forth in claim 1, wherein the metal element M is tungsten (W), and the composition formula is:

    aCaO·bTiO.sub.2 ·cLa.sub.2 O.sub.3 ·dZnO·eWO.sub.3.


6. 6. A dielectric ceramic composition as set forth in claim 5, which contains plate crystals.
 7. A method of preparing a dielectric ceramic composition in a shape, the method comprising the steps of:mixing oxides of metal elements which constitute a composition formula as recited in claim 1, and forming the resulting mixture into the shape; and firing the resulting shape at a firing temperature of 1,400 to 1,650° C.
 8. A method as set forth in claim 7, further comprising the step of reducing the firing temperature at a rate of 2 to 300° C. per hour after the firing at a temperature of 1,400 to 1,650° C.
 9. A dielectric resonator comprising:a resonator medium composed of a dielectric ceramic composition as recited in claim 1; and a pair of output and input terminals disposed on opposite sides of the resonator medium, whereby a high frequency signal is applied between the input and output terminals to resonate the resonator medium at a resonant frequency.
 10. A dielectric resonator comprising:a resonator medium composed of a dielectric ceramic composition as recited in claim 5; and a pair of output and input terminals disposed on opposite sides of the resonator medium, whereby a high frequency signal is applied between the input and output terminals to resonate the resonator medium at a resonant frequency. 